1. Field of the Invention
The present invention relates generally to the measurement of nuclear radiation using scintillation counters and, more particularly, to a method and apparatus for inhibiting radiation measurement during periods of excessive background radioactivity.
2. Description of the Prior Art
Scintillation counting techniques have been widely adopted to measure the activity of materials containing radionuclides. A scintillation counter typically comprises a scintillator and a photomultiplier tube. When radioactive disintegrations of the radionuclide of interest interact with the scintillator, light flashes (scintillations) are produced. The scintillations are detected by the photomultiplier tube and converted into electrical pulses having pulse heights corresponding to the energy of the scintillation being detected. A scaler counts the pulses to provide a measure of the radioactivity.
The most commonly used radionuclides, particularly in clinical and diagnostic procedures such as radioimmunoassay (RIA), are radionuclides which emit beta or gamma radiation. Counters for gamma radiation typically employ a scintillator crystal such as thallium-activated sodium iodide. Counters for beta radiation, on the other hand, are increasingly incorporating liquid scintillation techniques. In a liquid scintillation counter, a radionuclide-containing sample and a scintillator solute (e.g. p-Terphenyl) are disposed within a liquid solvent (e.g. p-Xylene). It is theorized that most of the kinetic energy from the nuclear disintegrations of the sample is absorbed by the solvent and then transferred to the solute which emits photons as visible light flashes or scintillations.
In any scintillation counter, it is essential to distinguish legitimate pulses produced by a sample or source being measured from background or noise pulses produced by other sources. Background radiation or noise results from a variety of sources including: (1) thermal emission within the photomultiplier tubes, (2) radioactive disintegrations from the materials of which the scintillation counter is constructed, and (3) cosmic energy surges which impinge on the counter. It should be noted at this point that samples may be typically counted for a period ranging from a small fraction of a minute up to several hours. Depending on the nature of the sample, the count rate may vary from less than 10 counts per minute (cpm) to many thousands of counts per minute. It is evident that for low count rates, even a small number of background counts can substantially alter the total pulse count and thus introduce major inaccuracies in the resulting measurements.
In view of the above, considerable effort has been expended to reduce the effect of background radiation in scintillation counters. In one approach, coincidence counters have been developed in which a scintillation must be coincidentally detected by two photomultiplier tubes to be counted. In another approach, the samples being measured have been surrounded by heavy shielding material, usually lead, to shield the sample from outside radiation. Other techniques have been devised to measure or approximate the background pulses introduced by the electronics of the measuring system and to subtract this background value from each resulting measurement.
However, the above advances have not been completely successful in eliminating the inaccuracies introduced by overloading energy surges produced by cosmic or other sources of noise. Such overloading surges typically comprise a large energy pulse followed by a string of smaller satellite pulses. Typically, the large pulse greatly exceeds the energy range being detected by the instrument. However, the satellite pulses typically have pulse heights which fall within the energy range or window of legitimate pulses from the sample being measured. Moreover, the satellite pulses are high frequency pulses, and thus the chance rate of coincidence between the satellite pulses is many times higher than that between random noise pulses. As a result, coincident counters do not successfully discriminate against the satellite pulses. Even with heavy shielding, the sample cannot be completely isolated from the effects of cosmic energy surges. Moreover, cosmic energy surges are random in nature and may occur from several times a minute up to 60 times a minute or more. Because of this unpredictability, it is difficult to approximate an associated background count for subtraction from the actual count.
My U.S. Pat. No. 3,859,532, assigned to the assignee of the present invention, discloses one approach for reducing counting errors introduced by overloading radioactive disturbances. The apparatus in the patent detects the occurrence of an overloading energy surge and inhibits the measurement of radioactivity thereafter for a predetermined time interval, for example 1 millisecond, approximating the anticipated duration of the energy surge. Unfortunately, however, at high sample count rates, legitimate sample counts which occur during the inhibit interval are lost, thereby introducing inaccuracies into the measurement.